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A Report on the R&D of the e-Bubble Collaboration. Colin Beal Virginia Polytechnic Institute and State University R.M. Wilson Saint Louis University Advisors Dr. Jeremy Dodd, Dr. Raphael Galea & Dr. Bill Willis Nevis Labs, Columbia University REU 2005. Some Neutrino Physics

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A report on the r d of the e bubble collaboration
A Report on the R&D of the e-Bubble Collaboration

Colin Beal

Virginia Polytechnic Institute and State University

R.M. Wilson

Saint Louis University

Advisors

Dr. Jeremy Dodd, Dr. Raphael Galea & Dr. Bill Willis

Nevis Labs, Columbia University

REU 2005


Outline

Some Neutrino Physics

Some Holes in Neutrino Physics

Goals of the e-Bubble Detector

Physics of the e-Bubble Detector

Test Chamber

Experimental Results

Simulation Results

Outline


Wolfgang Pauli, 1930

Cowan & Reines, 1956

Enrico Fermi

Using reactor source,

“neutrino”

-First experimental evidence of neutrino

(Italian for “little neutral one”)


Neutrinos

Weak interactors by the exchange of the W and Z bosons

http://www-numi.fnal.gov/public/images/standardmodel.gif


p

e-

Neutrinos

W

& Interactions with Matter

n

t

e-

W

e-

e-

Z

e-

n,p

Z

n,p

x


Neutrinos

t

… more interactions

e-

W

Z

e-

e-

e+

e-

W

W

x

e-

e+

e-


Neutrinos

& the Sun


Neutrinos

The Solar flux


Neutrinos

The First Solar Neutrino Detector

Homestake

  • Built at BNL in 1965

  • 615 tons tetrachloroethylene

  • Observed the following solar neutrino reaction…

  • Saw deficit in solar neutrino flux…

http://www.its.caltech.edu/~sciwrite/journal03/A-L2/greissl.html


Neutrinos

The Solar Neutrino Problem

The Solar Standard Model (SSM) is tested…

Super-Kamiokande

  • H2O Cherenkov Detector, 500 metric tons

  • Minimum ~3 MeV neutrinos

  • Detects Cherenkov light from scattered electrons

  • Reported 1/3 expected solar neutrino flux

http://ale.physics.sunysb.edu/nngroup/superk/pic/sk-half-filled.jpg

The missing neutrinos can be compensated for if a model incorporating new physics is taken into account…


Neutrinos

They Oscillate

Assuming that neutrinos do have some mass, and that their masses are a mixture of the neutrino (say ne and nm) flavor eigenstates…

Then the probability that an ne will be detected as an ne a distance L (km) away from its origin is given by…

constant

Energy of Neutrino (eV)

Mass difference


Neutrinos

The Solar Neutrino Solution

Sensitive to electron, muon and tau neutrinos…

SNO

  • D2O Cherenkov Detector, 1000 metric tons

  • Minimum ~3 MeV neutrinos

  • Detects Cherenkov light from scattered electrons

  • Reported expected solar neutrino flux

So what else is there to know?

http://www.pparc.ac.uk/Nw/Press/sudburysalt.asp


Neutrinos

There is so much more…

  • What can we learn from low-energy neutrino experiments? …

  • Most of the Suns power lies at energies well below the threshold of current real-time neutrino detection experiments.

  • Our models tell us that high energy neutrino oscillations (governed by the MSW effect) behaves much differently than low energy neutrino oscillations.

  • Is nuclear fusion the primary source of the Suns energy, or is there something else at work?

  • The neutrino magnetic moment m is much more accessible for measurement at low energies.


e-Bubble

The Objective

To design, build and implement a real-time low-energy neutrino detector* using a cryogenic liquid detection medium.

*The detector will be a tracking detector, i.e. one which utilizes the ionization track of electrons produced in a ne-e scattering event to extract information about the incident particle, in this case, a neutrino.


e-Bubble

Performance Goals

Due to the nature of low-energy neutrinos, we’ll need a detector with the following features…

  • Excellent spatial resolution (sub-mm)

  • Excellent energy resolution

  • Large volume or high event-rate

  • Low background


e-Bubble

Tracking Detector

2-D Detection Plane

Drifting Ionized Electrons

Incident Neutrino

n-e interaction

e-e ionizations


Neutrino-Electron Interaction

Origin of the Electron Track

Bahcall, John H., Rev. Mod. Phys., 59, 2, 1987.


Neutrino-Electron Interaction

  • Cross-Sections

    Magnetic Moment

m


Neutrino-Electron Interaction

  • Cross-Sections

    Weak Interactions


e-Bubble

Tracking Detector


e-Bubble

Information from Tracks

Length of Track

Energy of Neutrino

Total Ionized Charge

Origin of Neutrino

Shape of Track


e-Bubble

The Detector Medium

LNe

LHe

  • T = 27K

  • r = 1.24 g/cm3

  • ~1 metric ton

  • Short tracks ( 700 mm,  300 keV)

  • Pointing only for highest energy npp

  • Self-shielding

  • T = 2K

  • r = 0.125 g/cm3

  • ~5 metric tons

  • Long tracks (1-7 mm, 100-300 keV)

  • Good pointing capability

  • Minimum ionizing (low dE/dx)

  • Pure (long drifts, low internal background)

e-Bubbles


  • Solar npp flux 6.2E10 cm-2s-1

  • Expect ~674 ton-1year-1

LNe

  • T = 27K

  • r = 1.24 g/cm3

  • ~1 metric ton

  • Short tracks ( 700 mm,  300 keV)

  • Pointing only for highest energy npp

  • Self-shielding

  • Minimum ionizing (low dE/dx)

  • Pure (long drifts, low internal background)

e-Bubbles


e-Bubbles

… A Social Metaphor

A Red Sox fan enters Yankee Stadium…

Go home

r

And the “Red Sox Fan”-Bubble phenomenon may be observed…


e-Bubbles

In LNe (or LHe)

  • Equilibrium state of free electrons in Low-Z noble liquids (LHe, LNe)

  • Due to Pauli repulsion between free electron and noble atoms

  • ~1-2 nm diameter

  • Displaces ~50-100 atoms of liquid


e-Bubbles

In LNe (or LHe)

Useful Properties…

Creates large “drag” in liquid

Low mobility

Slow drift velocity in electric field

Small diffusion due to thermal equilibrium


LNe

Physics of Ionization Tracks

  • Two primary forms of charged particle energy loss…

  • Radiative (Bremsstrahlung)

  • Ionization


LNe

Physics of Ionization Tracks


LNe

Physics of Ionization Tracks


LNe

Physics of Ionization Tracks


LNe

Physics of Ionization Tracks


LNe

Physics of Ionization Tracks

250 keV Recoil Electron Tracks

150 keV Recoil Electron Tracks

(Single ionizations, parameterized angular distribution)


LNe

Pointing Capability

How well can we determine the origin of the incident neutrino?

  • Angular diffusion of the ionization track

  • Length of ionization track

  • Diffusion over drift in detector


LNe

Pointing Capability


LNe

e-Bubble Drifts

Liquid Surface

Einstein-Nernst Equation

for Thermal Diffusion

s

Path of e-Bubble Drift

Ionization Location


LNe

e-Bubble Drifts

Predicted Mobility…

Drift Velocity…

E = 1000 V/cm

E = 5000 V/cm


LNe

e-Bubble Drifts

Liquid Surface

What happens at the liquid surface?

Why does it matter?


LNe

Trapping e-Bubbles at the Liquid-Vapor Interface

  • Dielectric discontinuity at the interface (el> ev)

  • Potential well just beneath surface

  • e-Bubble has some probability of tunneling through potential barrier in time

Schoepe, W. and G.W. Rayfield, Phys. Rev. A, 7, 6, 1973.


LNe

Trapping e-Bubbles at the Liquid-Vapor Interface

Barrier Height


2-D Detection

Ejecting Charge from Liquid Surface

  • Method needs to be conducive to maintaining resolution (energy and spatial)

  • Local high-field pulsing at surface

  • Photo-emission

Due to their large size, e-Bubbles are highly sensitive to photo-excitation.

Effective, but noisy


2-D Detection

Charge Amplification

Due to low ionized charge, a method of amplification is required…

GEMs

  • High localized fields

  • Charge amplification and light emission (~1000x amplification)


2-D Detection

Charge Amplification

Due to low ionized charge, a method of amplification is required…

GEMs

  • Commercial CCD Cameras to read out light emission

  • Pixelated anode

  • No method for in-liquid detection found effective

Garfield simulation of charge amplification and drifts


In the mean time…

some proof of principle.

  • Experimental verification of LNe physics

  • Simulated LNe drifts

All essential in constructing a large scale detector


Research and results
Research and Results

  • Outline:

    • e-Bubble Test Chamber Setup

    • Experimental Data

    • Computer Simulation Results


Experimental run design
Experimental Run:Design

e-Bubble experiment is set up at Brookhaven National Lab

A cryostat uses liquid

Helium (~4K) and liquid

Nitrogen (~77K) to cool

the test chamber.

Optical windows enable

“first-hand” observation

of the experimental runs


Experimental run test chamber setup
Experimental Run:Test Chamber Setup

Electrons must be “artificially” inserted

into the test chamber

  • Goals:

    • - Test electron sources

    • - Make electron bubble drift measurements


Experimental run electron sources
Experimental Run:Electron Sources

  • Photo-Cathode

  • High Voltage Tip

  • Radioactive Alpha Source


Experimental run drift time
Experimental Run:Drift Time

Experimental

Theoretical

Although the experimental drift time differs from the predicted time by only a few ms, many approximations were used.

…stay tuned

Drift time is 78 ms @ 4 kV/cm

Using µ = 1.6E-3 (cm2/Vs)

Drift time is ~80 ms @ 4 kV/cm


Experimental run mobility
Experimental Run:Mobility

  • Using the predicted drift time equation, mobility was fitted as a free parameter

    1.66E-3 < µ < 1.9E-3

    (cm2/Vs)

  • The derived mobility was consistent with previously determined electron bubble mobility in LNe (Storchak, Brewer and Morris).

Drift time (ms)

E-Field (kV/cm)

Drift time (ms)

E-Field (kV/cm)

C (cm2/V) is a constant to compensate for

omitting the emission and anode regions


Experiment run drift velocity
Experiment Run:Drift Velocity

  • The electron bubble drift velocity can be determined using:

    V=µE

    For µ=1.6E-3 (cm2/Vs) and E=4 kV/cm;

    V = 6.64 cm/s.


Experiment run tip charge emission
Experiment Run:Tip Charge Emission

  • The total charge deposited is calculated using

where; Q is the total charge at the anode (MeV), q is the charge injected by pulse (MeV), A is the measured amplitude (mV), a is the calibrated pulse voltage (mV), ∆T is the measured signal FWHM (ms), and ∆t is the calibrated signal FWHM (ms).

q = 10 MeV, a = 14:6 mV and t = 0:222 ms.


Experiment run mesh transmission
Experiment Run:Mesh Transmission

  • The meshes in the test chamber will stop many electron bubbles.


Experimental run trapping time
Experimental Run:Trapping Time

  • The first attempt at measuring the electron bubble trapping time at the liquid-vapor interface in LNe was inconclusive.


Experiment run conclusions
Experiment Run:Conclusions

  • Photo-Cathode in LNe= Bonk!

  • High Voltage Tip= Success!

  • Drift Time = 76 ms (under a 4 kV/cm drift field)

  • Drift Mobility = 1.66 x 10-3 (cm2/Vs)

  • Drift Velocity = 6.64 cm/s

  • Tip Charge Emission

  • Mesh Transmission


Simulations
Simulations

  • Garfield:

    • Cell Definition

    • Gas Definition

    • Field

    • Drift

    • Signal


Simulations drift time mobility
Simulations:Drift Time; Mobility

76.85 ms

  • Electrons were drifted through the simulated cell by defining mobility=1.9E-3 (µ=1.9E-3 cm2/Vs).

  • Recall the experimental drift time was ~78 ms.

The predicted drift time

is 78. 5 ms (µ=1.9E-3

cm2/Vs)


Simulations diffusion
Simulations:Diffusion

LongDiff = .001, TransDiff=1E-5 (cm/cmdrift)

  • Diffusion (longitudinal and transverse) effects the result of the simulated drifts.

  • Generally, as diffusion increases the observed signal will widen and exhibit a more predominant tail

LongDiff = .001, TransDiff=1 (cm/cmdrift)


Simulations diffusion1
Simulations:Diffusion

  • Diffusion displays a “threshold” characteristic.


Simulations signal
Simulations:Signal

Signal resulting from 80 electron bubble drifts


Simulation conclusions
Simulation:Conclusions

  • Consistent drift time results.

    • Yields accepted electron bubble mobility and velocity

  • Diffusion “threshold” characteristic

  • Simulated signal for direct comparison to experimental data


What now
What now?

  • Little Picture:

    • Trapping Time

    • Gas Bubbles

    • GEM Characteristics

  • Big Picture

    • Finish Research and Design

    • Ramp Up

    • Construction



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